Fig. 2. Relations between ischemic duration and neuronal damage in all experimental animals.  Rectangles and  circles represent data for animals in the nitrous oxide group and nitrogen group, respectively. The logistic regression curves show close relations between ischemic duration and neuronal damage (nitrous oxide group,  line A :  r ²= 0.59,  P < 0.0001; nitrogen group,  line B :  r ²= 0.91,  P < 0.0001). The 95% confidence intervals  (shaded areas ) did not overlap from 3.07 to 6.63 min of ischemic duration (*). Ischemic durations necessary for causing 50% neuronal damage in the nitrous oxide group and nitrogen group were 4.45 and 5.27 min, respectively.